For example, when observing the inside of a living body, with microscopes, because the outer diameter of an objective lens is large, it is necessary to make a large incision in the living body for observation. However, making a large incision in the living body leads to high invasiveness, thus imposing an inordinate burden on the living body and making it impossible to perform observation for a long time. Furthermore, with general endoscopes, because the outer diameter is small, the invasiveness with respect to the living body is low, but, because the magnification is low, it is inadequate for observing the behavior of body tissue or molecules in a cell. Furthermore, with magnifying endoscopes, the magnification is high, but the numerical aperture (NA) on the object side is low, thus resulting in a lack of resolution and luminance.
Thus, as an objective optical system with which desired observation can be performed while achieving low invasiveness, for example, PTL 1 discloses an objective optical system in which the outer diameter at a distal end is relatively small, and the numerical aperture is large. According to this objective optical system, a small hole is formed in the living body, and the objective optical system is made to pass through this hole, thereby making it possible to observe the living body with low invasiveness.